The role of prostaglandins in renal physiology remains unclear. In the last year we have shown that there is a circadian rhythm for urinary prostaglandin E excretion (iPGE) in normal women and that this is an essential consideration in the design of any acute experiments. Alterations of posture at the same time of day reveal a significant fall in urinary iPGE excretion when the subject stands. Alterations of dietary sodium and potassium intake had no effect on urinary iPGE excretion. These procedures have been shown to have a pronounced effect on the renin-angiotensin-aldosterone system. Therefore, iPGE excretion may be independent of the renin-angotensin-aldosterone system in normals. Aspirin administration to patients with lupus nephritis caused a fall in glomerular filtration rate which parallels the fall in urinary prostaglandins. This suggests that prostaglandins may have an important role in regulation of glomerular filtration rate in patients with lupus nephritis. Prostaglandins are important in the pathophysiology of Bartter's syndrome. Urinary iPGE excretion is elevated in patients with this syndrome and treatment with prostaglandin synthetase inhibitors reversed the abnormalities of the syndrome including hyperreninemia and hypokalemia. The exact mechanism of action of prostaglandins in this disorder is as yet unexplained.